This invention relates to improvements in sewage treatment systems, and, more particularly, to improvements in sewage treatment systems of the type in which a clear effluent is obtained by a finishing treatment of a sewage effluent stream which has previously been subjected to a primary and a secondary treatment.
In locations not served by municipal sewage systems, septic tanks, settling systems and other small scale sewage treatment systems are widely used. Separate sewage treatment systems for use, for example, for a house, small apartment complex, or the like, are becoming increasingly popular. Such systems commonly comprise a first settling tank for separating the solid matter from the sewage, a second tank where the sewage, both liquid and solid, is exposed to oxygen and decomposed by an aerobic action and a third tank for final separation by settling. The effluent from the third tank is not always as clear as it should be, and commonly has a minor proportion of solids and an objectionably high biochemical oxygen demand (BOD). It is desirable to treat such an effluent further, both to clarify it as well as to reduce its BOD.
Apparatus conventionally used for the finishing treatment of such an effluent commonly comprises a spray-recirculation tank in which the effluent is recirculated through a spray header to aerate, usually intermittently, and a separate filtration unit through which the aerated effluent is pumped. Such a two-unit system usually requires two pumps.
Another presently used system utilizes a single spray-recirculation tank containing the filtration unit. Incoming effluent is filtered through the filtration unit and a submersible pump spray-recirculates the filtered liquid, usually intermittently. An outlet for the effluent is located at a desired level usually about one foot from the bottom of the tank so as to maintain the liquid level at this height. The disadvantage of this system is that, except where the unit is located on an elevated embankment of the like, the outlet is several feet below the specified discharge level, and therefore, requires a second pump.
The apparatus of this invention combines the simplicity and economy of the single unit, and at the same time, obviates the cost and problems associated with a second pump. Moreover, the raised outlet of the tank permits an accumulation of a surge until such time as the pump should start up, thus permitting better aeration of the liquid in the tank. It will be recognized that where the outlet level, as in the prior art, is situated at the liquid level, any surging of liquid would immediately result in a flow of liquid from the tank.